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Cretaceous Period

Pterosaurs were discovered many years before the term dinosaur became a household word. The first scientific description of a pterosaur fossil was written in 1784 for a specimen discovered in Germany. The specimen was found in the limestone quarries of Solnhofen, the same general area in which Archaeopteryx, the first bird, was discovered more than 75 years later. The pterosaur in question was Pterodactylus (Late Jurassic, Germany) however, it was not named until 1812, when French anatomist...

Unlike most non-avian dinosaurs, which had reptilelike skin composed of nonoverlapping scales, pterosaurs had leathery skin more like that of mammals. From the earliest discoveries of pterosaurs, scientists wondered whether this skin may have been covered with an insulating layer of fur or feathers. Such a coating would have helped the creatures retain body heat, an important advantage for an active flyer. In 1970, a fossil of a small pterosaur provided possible evidence that these creatures...

While the prevailing and best-received theory of bird origins is that the first birds evolved from non-avian maniraptoran dinosaurs, there are two competing ideas that need mentioning. The first idea, long held by a few paleontologists, including Alan Feduccia and Larry Martin (b. 1943), is that birds and maniraptorans evolved at the same time from a common archosaurian tetrapod ancestor yet to be discovered. That idea would most likely depend on a trees-down origin of powered flight in birds....

Sauropods were the largest of all dinosaurs, pushing the anatomical, physiological, and metabolic extremes of terrestrial vertebrates to the upper limits. Sauropods included the tallest, heaviest, and longest land animals to ever walk the Earth. Sauropods were members of the clade known as Sauropodomor-pha (lizard foot form). This clade is part of a larger clade the Saurischia. Evolutionary adaptations in body size have never been pushed to the anatomical, physiological, and metabolic extremes...

Ichthyosaurs the fish lizards were some of the first fossil creatures to attract scientific attention. An illustration of one was published as early as 1699, but the bones were believed to be those of a fish. It was not until 1814 that the renowned French anatomist Georges Cuvier compared the skull and skeletal features of those fossils to the bones of other vertebrates, thereby proving that ich-thyosaurs were related to reptiles. This was a startling conclusion to make at the time, during the...

Like all of the non-avian dinosaurs, sauropods perished at the end of the Late Cretaceous Epoch. Reasons for the mass extinction of dinosaurs and many other fauna at that time are explored in the conclusion of Last of the Dinosaurs. As for the sauropods, their long history, with roots in the Late Triassic Epoch, is one of tremendous success and adaptive persistence. Aside from body size, skull and tooth shape, and the relative length of the forelimbs and neck, sau-ropods shared many...

Carnivorous dinosaurs were members of the saurischian clade known as Theropoda (beast foot), which included numerous distinct subgroups all sharing a common ancestor. About 40 percent of all known dinosaur taxa were theropods. Their remains have been found on every continent, and theropods ranged in size from the tiniest of dinosaurs to the largest terrestrial predators that ever existed. Another book in this series, Dawn of the Dinosaur Age, provides a detailed look at the classification of...

Powered flight is not unique to birds. Insects, bats, and pterosaurs also evolved the ability to fly under their own power. Among animals with backbones, powered flight has evolved three separate times first in the pterosaurs, then in birds, and finally in bats each independent of one another. This is an example of convergent evolution, wherein different organisms evolve similar adaptations, often in response to analogous environmental stresses or opportunities. There are three main anatomical...

The clade of theropods known as Avialae is a natural group including all of the descendants of the common ancestor of Archaeopteryx and Passer domesticus, the modern house sparrow. The basal members of this clade, discussed below, consist of early avians that existed prior to the close of the Mesozoic Era. These creatures represent the transitional stages of evolution from Archaeopteryx to modern birds. Anatomical traits that unite these avian theropods include long forelimbs a pointed...

The earliest theropods the Coelophysoidea lived during the Late Triassic and Early Jurassic Epochs and represented the first radiation of carnivorous dinosaurs. Several other primitive lines of theropods persisted well into the Late Cretaceous, particularly in regions of the Southern Hemisphere (now Argentina, North Africa, India, and Madagascar). These were the ceratosaurs, many of which exhibited bizarre adaptations not seen in theropods of the Northern Hemisphere. The body plan of most...

The Archaeopterygidae is a group including only the first known bird, Archaeopteryx, from the Late Jurassic of Germany. Archaeopteryx is represented by several excellent, two-dimensional flattened fossil skeletons, many of which include skulls and or feather impressions. Archaeopteryx has been called the most valuable of all fossils because of its importance in understanding the evolution of species. At the time of its discovery in the early 1860s, Charles Darwin had only recently proposed his...

Another group of reptiles from the Mesozoic that survives to this day are the turtles. Their origins reach back before the Age of Dinosaurs, to the end of the Permian Period and the Early Triassic Epoch, more than 210 million years ago. The first forms of turtles were land animals. Marine turtles first appeared in the Early Cretaceous Epoch. Along with mosasaurs, they were among the last group of the great marine reptiles to take to the oceans. All species of Mesozoic marine turtles are now...

Sauropods were the dominant herbivorous dinosaurs of the Jurassic Period. The dwindling of sauropods in the Northern Hemisphere during the Cretaceous Period has been attributed to the rise of flowering plants, for which sauropods may have been poorly equipped to eat, and to the success of various ornithopods. Iguanodontids and hadrosaurs developed some of the most sophisticated and adaptable dental apparatuses ever seen in plant-eating vertebrates. They were well-equipped for eating the tough...

Ornithischian dinosaurs made up one of the two major clades of dinosaurs. The ornithischians were united, among other features, by a generalized design of the pelvis known as a bird, or ornithischian, hip. Note that the ornithischian bird hip has only a superficial resemblance to actual birds saurischians discussed in the previous chapter the two kinds of dinosaur hips are thought to have arisen independently. Including a variety of bipedal and quadrupedal herbivores, ornithischians were highly...

All tetanurans other than the Spinosauroidea were part of a group of related theropods named the Avetheropoda. The origins of this group date from the Middle Jurassic, and the group consisted of several diverse subgroups representing, among others, the smallest known dinosaurs as well as some of the largest. It is also within the Avetheropoda bird theropods that the first definitive link between dinosaurs and modern birds is seen. In fact, the clade Avetheropoda was defined by American...

The Coelurosauria represent a widely diverse group of theropods ranging in size from the tiny Microraptor and Compsognathus to the enormous Tyrannosaurus. The name Coelurosauria means hollow-t ail lizards. When the name was proposed in 1920, the group was intended to include mostly small predatory dinosaurs that had lightweight skeletal features. The category became a dumping ground for many difficult-to-classify theropod taxa that were not part of a naturally related group. The Coelurosauria...

Birds are dinosaurs just as snakes are reptiles and horses are mammals. The most prominent feature that distinguishes birds from other dinosaurs is their ability to fly. Last of the Dinosaurs uses the term non-avian dinosaur to refer to those dinosaurs that did not fly and are thus not considered true birds. The basal Avialae, or primitive birds, described in this chapter are defined as maniraptorans closer to birds than to Deinonychus, a dromaeosaurid. Some of these creatures flew and some did...

This chapter described widespread changes to ocean and land environments, including changes to climates and the rise of flowering plants, that served as catalysts for the spread of dinosaurs and other vertebrates. 1. Continental drift during the Cretaceous Period caused the continued breakup of the northern and southern supercontinents of Laurasia and Gondwana respectively, separating the continents into landmasses approximating their present-day configuration. 2. There was a dramatic rise in...

I want to thank the many dedicated and hardworking people at Chelsea House and Facts on File, some of whom I know but many of whom work behind the scenes. A special debt of gratitude goes to my editors Frank Darmstadt, Brian Belval, Justine Ciovacco, Lisa Rand, and Shirley White for their support and guidance in conceiving and making The Prehistoric Earth a reality. Frank and Brian were instrumental in fine-tuning the features of the series as well as accepting my ambitious plan for creating a...

The oversized skulls of pterosaurs varied widely from one taxon to another. The most typical pterosaur skull shape was long and narrow with a pointed snout, nostrils close to the eyes, and often a dazzling array of pointed teeth for spearing fish. Several others had jaws that were unique even among pterosaurs. The following is an overview of various jaw adaptations found in pterosaurs and representative taxa that possessed such adaptations. Short, widely spaced, pointed teeth. In some...

Ceratopsians and other marginocephalians were highly abundant animals. They represent about one-quarter of the dinosaur taxa discovered from the latest Cretaceous Period. Together with the iguanodontian ornithopods, these dinosaurs successfully displaced sauropods as the dominant large herbivores in the Northern Hemisphere. Like the success of the ornithopods during the same time, marginocephalians may have succeeded where sauropods failed in adapting to the consumption of a more diverse...

The Middle and Late Jurassic Epochs were evenly warm and temperate across much of the globe. This is not to say that the world had only one uniform climate during the Jurassic. Just like today, there were Jurassic deserts, Jurassic mountains, Jurassic islands, and other regions, each of them with their own unique climatic regimes. Although the poles were warmer than they are at present, they were still cooler than equatorial regions. The submergence of large portions of continental landmasses...

The story of predatory dinosaurs began during the early stages of dinosaur evolution with the appearance of the first theropods in the Late Triassic Epoch. By the Middle and Late Jurassic Epochs, the evolutionary lineages of meat-eating dinosaurs had diversified dramatically, leading in one direction to such large carnivores as Allosaurus Late Jurassic, western North America and in another direction to such small, chicken-sized insectivores as Compsogna-thus Late Jurassic, Germany . The Late...

One of the most familiar images in the folklore of dinosaurs is a battle to the death between Tyrannosaurus and the horned dinosaur Triceratops Late Cretaceous, western North America . Triceratops was a large horned dinosaur, one of the last of its kind, and probably would have made a formidable foe for T. rex had the theropod been careless enough to engage it in a tussle. Weighing as much as 6 tons 5.4 tonnes , a defensive-minded Triceratops had many advantages to draw upon in battle. Its...

Sexual dimorphism occurs when male and female individuals of a single species exhibit different anatomical traits. Such traits can be determined reliably in dinosaurs only when an abundance of specimens from the same species can be compared. Such traits helped dinosaurs distinguish male individuals of their species from females, and may have also been related to particular behaviors. For example, male African elephants have tusks, while females do not. These tusks are used during combat or...

The sauropod group Neosauropoda is subdivided into two major clades the Diplodocoidea comprising the Rebbachisauridae, Di-craeosauridae, and Diplodocidae and the Macronaria composed mainly of the Camarasauridae, Brachiosauridae, and Titanosauria . Of these, only the Macronaria had taxa that survived into the Early and Late Cretaceous Epochs as members of the brachiosaurs and titanosaurs. The Macronaria were distinguished from other sauro-pods by skull features that included a nasal opening that...

The Hesperornithes were a highly specialized subgroup of aquatic ornithuromorphs. Also loonlike but probably all flightless, these large birds measured up to 3.3 feet 1 m long and were divers. There are about nine valid taxa of Hesperornithes and they are definitely known only from fossil localities in the Northern Hemisphere. The best-known hesperornithean is Hesperornis Late Cretaceous, western North America , for which several skulls and partial skeletons have been found. When it was first...

The Cretaceous Period, which spanned 80 million years, was the last great time span of the Mesozoic Era. Coming to a crashing climax 65.5 million years ago, it marked the last reign of the dinosaurs and their reptilian cousins in the oceans and in the air. The name Cretaceous is Latin for chalk and refers to the abundant layers of chalk deposits that are found in sedimentary rocks of Europe that date from this period. The Cretaceous Period is further subdivided into two epochs the Early...

The Carnosauria is a group of mostly large theropods measuring 16 feet 5 m long or more and characterized by massive heads burly, powerful necks relatively short arms with three fingers per hand powerful hind limbs and long, deep tails. Except for the spinosaurs and tyrannosaurs, the largest theropods come from the ranks of the carnosaurs. Carnosauria includes three subgroups the Sinraptori-dae, the Allosauridae, and the Carcharodontosauridae. The Sinraptoridae were large predators that...

The last surviving group of sauropods were titanosaurs, and their distribution was nearly worldwide. Titanosaurs had roots in the Late Jurassic of Tanzania, Thailand, and western North America. Although specimens have been found all over the world, most titanosaur remains are fragmentary and lack skull elements. The most complete specimens have been found in Argentina and Madagascar, where the recent discoveries of two exquisite skulls have enabled paleontologists to better understand the...

The Spinosauroidea were a group of related theropods considered to be basal within the Tetanurae. The group is further broken down into two smaller groups. The Megalosauridae were medium- to large-sized theropods that mostly lived during the Middle and Late Jurassic Epochs. The Spinosauridae included several large-bodied forms that reigned as the largest carnivores in their ecosystems up until the early part of the Late Cretaceous, at which time they were replaced by more derived tetanurans....

The crested lambeosaurine hadrosaurs displayed an enormous variety of helmets, hatchets, and hood ornaments on their skulls. As a functional piece of anatomy, the crest was part of the nose. Fossil hadrosaurs, for which a cross-section of the skull can be observed, show that the nostrils were connected to the throat by a hollow set of cavities and tubes through which the dinosaur breathed. Most other kinds of dinosaurs did not have such crests, however, so we may assume that hollow crests had a...

The braincase of a pterosaur skull holds clues to the many connections between the brain and other parts of the body. Evidence of these nerve connections can be seen in the form of holes in the braincase through which nerves were once threaded to attach the brain to other organs. One of the first scientists to study the structure of brains in extinct creatures was Tilly Edinger. In 1927, she published an important study of the brains of pterosaurs based on casts of their brain cavities. She...

Today's crocodiles are what remain of a long line of reptiles that first appeared during the Middle Triassic Epoch. Only a few kinds survive today, including crocodiles, alligators, and gavials. Forms of crocodiles that lived in the ocean were highly successful during the Jurassic and Cretaceous Periods. They returned to the sea, as did other forms of reptiles, to take advantage of abundant food supplies in the form of fishes, cephalopods, and other marine reptiles. Nearly all marine crocodiles...

Dinosaurs hatched from eggs, like their bird descendants and most living reptiles. Hundreds of dinosaur egg sites have been discovered three-quarters of these are in North America and Asia. Most of the sites that have been found date from the Late Cretaceous Epoch, and many are associated with hadrosaurs. Knowledge of the lifestyle and behavior of ornithopod dinosaurs has been greatly enriched by the discovery of their fossilized eggs, nests, and young. This is largely due to the pioneering...

The most primitive members of the Ceratopsia are represented by Yinlong, a group called the Chaoyangsauridae, the Psittacosauridae, and a number of basal members of the Neoceratopsia, most notably the protoceratopsids. Most basal Ceratopsia have been found in Asia, whereas the larger, more derived horned dinosaurs all hail from western North America. Yinlong hidden dragon is a basal ceratopsian from the Late Jurassic Epoch of China. It was small and primarily bipedal, measuring only 4 feet 1.2...

Marine turtles evolved several features to improve their ability to live in the ocean. The most obvious changes were to develop a more lightweight, streamlined body that could propel itself in the water. The shell of a turtle has two parts. The top part, protecting its back, is called the carapace. The bottom part, protecting its belly, is the plastron. The shells of land-dwelling turtles are designed to enclose the body in a tightly knit mosaic of bony plates. This was an effective form of...

Vertebrate Evolution An interactive diagram of vertebrate evolution with links to example fossil specimens in the world-famous collection of this museum. vertebrate An extensive visual guide to Australian fossils. Bernard Price Institute For Palaeontological Research, University of the Witwatersrand, Johannesburg. Fossil Picture Gallery Information for a wide variety of South African vertebrate fossils. Carnegie Museum of Natural History. Dinosaurs in Their...

An abundance of fossilized skin impressions have been associated with specimens of hadrosaurs. These trace fossils left the pattern of the animal's skin in the mud where a given dinosaur died. All of the evidence uncovered so far regarding the skin of unfeathered dinosaurs shows that they had nonoverlapping scales similar to those of the modern monitor lizard. These scales varied in size across different parts of the body. The scales were generally smaller, for flexibility, around the head and...

Although the head was generally more lightly built with a long, narrow snout, extinct marine crocodiles resembled today's crocodiles in many ways. Here are some of the anatomical features that marine crocodiles share Long, narrow skull. The skulls of marine crocodiles were lightweight and had a long, narrow snout. When viewed from above, they had an extremely elongated triangular shape. Eyes on the sides. The eyes of marine crocodiles were on the sides of the skull rather than the top as in...

The discovery of fossils gives us the only hard facts about the distribution of dinosaurs across the Mesozoic Earth. As such, an understanding of dinosaur distribution is strongly dependent on the availability of fossils, which is subject, in turn, to the availability of fossil deposits of Mesozoic age. The time of the dinosaurs spanned an enormous gulf in time 164 million years composing the better part of three long geologic periods the Triassic, the Jurassic, and the Cretaceous. There is a...

The first vertebrates to achieve powered flight were the Pterosauria, the winged lizards. Their rise and success paralleled that of the dinosaurs and lasted as long. Pterosaurs mastered the air in many forms and ranged in size from that of a small bird to the largest vertebrate ever to soar through the skies, Quetzalcoatlus Late Cretaceous, Texas , a creature with a wingspan of 40 feet 12 m . Pterosauria, the flying reptiles, were not dinosaurs. They were also not related to birds. All...

The evolution of highly specialized, increasingly robust and varied horns and frills in ceratopsians begs the question of their utility and function. No one doubts that horns and frills were used for protection. The ceratopsian frill provided protection for the neck, normally one of the most vulnerable parts of the body to be targeted by predators. Horns on the nose and brow were undoubtedly dangerous weapons when thrust into action by an angry ceratopsian. Why, however, was there such a great...

The significance of the iguanodonts to the history of dinosaur science is a matter of record. In the year 1822 19 years before the term dinosaur became a part of scientific literature a physician and amateur geologist named Gideon Mantell 1790-1852 was visiting a patient near Lewes, in the English countryside. Accompanying him was his wife, Mary Ann. As the story goes, while the doctor tended to his patient, Mary Ann took a stroll along a country road, where she noticed a jumbled pile of...

The sharp, conical teeth of mosasaurs were able to cut through or crush most any kind of fleshy or bony creature they could fit inside their mouth. Mosasaurs probably hunted by lying in wait and ambushing their prey. Some mosasaurs had a bony rod at the end of the snout, and it has been suggested that they used it to ram their prey, stunning them. All mosasaurs shared the following anatomical traits Many sizes. The largest mosasaurs, including Mosasaurus and Tylosaurus knob snout lizard , could...

All sauropods had a generally similar body plan featuring a small head, a long neck, a huge body, quadrupedal posture, and a long tail. The presence of pneumatic concavities in the spinal bones of sauropods provided lightness without sacrificing strength. The presence of pneumatic concavities in the spinal bones of sauropods provided lightness without sacrificing strength. Traits that united the sauropods included sturdy, upright limbs to support their massive weight four or more fused or...

The advanced iguanodontian ornithopods known as the Hadrosau-ridae bulky lizards include some of the best-understood dinosaur taxa because of the abundance of their specimens and often because of excellent preservation. Hadrosaurs evolved some of the most effective methods for eating plants ever seen. These dinosaurs had more teeth than any other known land animals. There are more than 35 scientifically accepted taxa of hadrosaurs. Hadrosaurs have also played an important role in the...

This chapter explored the evolution and lifestyles of the Margino-cephalia. 1. The Marginocephalia were ornithischian dinosaurs that included the Ceratopsia horned faces and Pachycephalosauria thick-headed lizards' . They were a subgroup of the Cerapoda, the same group that included the ornithopods. 2. The name Marginocephalia refers to a bony shelf at the back of the skull that is characteristic of all members of the clade. 3. Marginocephalia are found only in the Northern Hemisphere and...

The Confuciusornithidae is another small clade of primitive birds, but it is represented by hundreds of complete specimens of its best-known taxon, Confuciusornis Early Cretaceous, China . All three known taxa of confuciusornithids were discovered in the exquisite fossil deposits of Liaoning and neighboring provinces in northeastern China. These basal birds were closer to modern birds than was Archaeopteryx and had already shed most of the traits of non-avian dinosaurs teeth and a long tail...

With their large heads and streamlined bodies, pliosaurs were probably capable of quick movement and sustained chases at high speed. They were pursuit predators, but not as fast as ichthyosaurs. They would chase large prey until they could grab them in their powerful jaws. Pliosaur teeth were sturdy, cone-shaped, and sharp, capable of cutting through thick flesh and even bones. Their mouths were huge, making any sea creature of the time a possible meal, including the long-necked plesiosaurs,...

Mosasaurs retain many of the attributes of their terrestrial ancestors. Unlike ichthyosaurs and plesiosaurs, which are presumed to have had smooth skin, mosasaurs still had reptilian scales. Their ancestry can be traced to ancient snakes, although for many years some paleontologists have thought that mosasaurs were more closely related to extinct monitor lizards. Mosasaur species ranged in size from the length of a minibus to that of the longest tractor-trailer, from about 10 to 57 feet 3 to...

The Plesiosauria were more fully adapted to the ocean than the Nothosauria. It is difficult to imagine that they willfully crawled out of the water except when they were still young and small. Imagine how clumsy they would have been on land, dragging themselves along on their paddles. It would have made them extremely vulner--able to dinosaurs and other terrestrial predators. Even the sturdy belly ribs of a 30- to 50-foot 9 to 15 m plesiosaur or pliosaur might not have been strong enough to...

Reptiles had already found success on land when some taxa began to explore the oceans again. Life in the ocean required good swimming skills so that marine reptiles could catch prey and escape predators. Marine reptiles adapted to ocean life by optimizing anatomical traits that reduced drag and provided them with speed and maneuverability in the water. Features such as webbed or flipperlike limbs and a vertical flattening of the tail aided in locomotion and could have also helped them to dive...

The group of marine reptiles known as the Plesiosauria dominated the oceans of the Mesozoic for about 140 million years, up until the last days of the dinosaurs. Nested within the Plesiosauria were the subgroups Plesiosauroidea, the long-necked plesiosaurs, and the Pliosauroidea, the short-necked plesiosaurs. The Nothosauria, also discussed here, were a sister group of long-necked marine reptiles that evolved before the plesiosaurs. It is interesting to note, however, that at just about the...

Because placodonts were adapted for living in shallow near-shore ocean environments, their anatomy was optimized to help them plod along the seafloor looking for stationary prey such as hard-shelled mollusks and crustaceans. Placodonts fall into two body types the unarmored placodonts, informally known as the Placodontoidea, and the armored placodonts, known as the Cyamodontoidea. Body characteristics of the placodonts included Skull. The placodont skull was strong and heavily built. It was...

Ichthyosaurs were the most fishlike in appearance of all marine reptiles, but this resemblance is superficial because the two kinds of animals are not related. The flippers of ichthyosaurs and other marine reptiles are a highly modified form of foot once used to walk on land. The bones are thick and still show the evidence of having once consisted of a wrist or ankle and fingers or toes. Anatomical traits of ichthyosaurs included Streamlined body. Except for the earliest forms, ichthyosaurs had...

Last of the Dinosaurs begins by looking at the geological and ecological conditions that created opportunities for the expansion of dinosaurs of the Early and Late Cretaceous Epochs. Section One encompasses worldwide geologic and climatic changes of the Cretaceous Period. Chapter 1 describes widespread changes to ocean and land environments, including changes to climates worldwide, that served as catalysts for the radiation of dinosaurs and other vertebrates. Temperate global climates allowed...

Until the Early Cretaceous Epoch, the world's flora was dominated by ferns and gymnosperms seed plants whose seed embryos are not protected by a fruit, cone, or other body. Gymnosperms first appeared in the late Paleozoic Era and became dominant during the first half of the Mesozoic Era. They are still represented today by more than 600 known species of conifers evergreen trees , cy-cads, gnetophytes, and Ginkgo, none of which have flowers or fruits. Gymnosperms are typically tough and hearty....

Marine reptiles were air-breathing creatures that lived in the water. Their ancestors were land-dwelling reptiles from the Permian and Triassic Periods. Except for the marine crocodiles, they were not closely related to dinosaurs. Some marine reptiles trace their ancestry to the roots of the reptilian evolutionary tree, while others were close relatives of lizards and snakes. Like today's turtles and crocodiles, they would hold their breath while under water. Reptiles first dominated life on...

Nothosaurs represent an early attempt of land-dwelling reptiles to find a home in the sea. Except for their webbed feet and long, streamlined necks, the earliest nothosaurs resemble lizards that simply took to the water to find food. Although the last of the nothosaurs had evolved paddles that made them better swimmers, they still moved about in the water by undulating their bodies from side to side. Their tail was the main source of their propulsion. Their shoulder and hip bones never changed...

The question of thermoregulation in dinosaurs has been the source of lively debate, but there is more agreement among paleontologists when it comes to the flying reptiles. From the earliest days of pterosaur study, scientists have concluded that these creatures were endothermic warm-blooded , not ectothermic cold-blooded . Warm-blooded creatures have an internal source of body heat regulated by their heart and circulatory systems. They are not dependent on the environment as their source of...

Whereas the Jurassic Period was relatively quiet geologically, the geography of the Cretaceous Period witnessed a dramatic resurgence of tectonic plate movements. Continental drift during the Cretaceous Period caused the continued breakup of the northern and southern supercontinents of Laurasia and Gondwana, separating them into landmasses approximating their present-day configuration. The northern Laurasia included areas that became North America, Europe, and Asia. The southern landmass of...

The Pachycephalosauria thick-headed lizards consisted of small to medium-sized bipedal, ornithischian herbivores that measured in length up to about 15 feet 4.5 m . They are the only dinosaur group distinguished by a thick, bony skull cap. Of the 14-plus recognized genera of pachycephalosaurs, nine are known only from skulls and skull fragments. Only a single new taxon of pachycephalosaur has been named in the past 25 years Sphaerotholus, in 2002 , indicating the relative scarcity of this clade...